In the iron- and steel-making industry the blast furnace work has recently come to face serious difficulties as the result of the exhaustion of coal resources which, in turn, has caused deterioration of coal quality, a great rise in the price of high quality coals and difficulties in obtaining such coals. Thus, the production of coke by means of compounding coals has been adopted as a supplementary measure for improving the quality of coals used in the production of blast furnace cokes in order to overcome the insufficiency in strongly caking coal. In this case, however, it is necessary to improve the apparatus for the production of coal briquettes, namely, to improve the performances of the briquetting machine.
In a briquetting machine, a weakly caking coal or a noncaking coal is used either in place of or together with the aforesaid strongly caking coal or caking coal. A weakly caking or noncaking coal is finely pulverized with or without strongly caking or caking coal to such degree that at least 90% thereof is of a particle size of 3 mm of finer, the pulverized coal is kneaded together with a caking agent and cooled, and then the kneading mixture is briquetted into an ellipsoidal shape having a longer diameter of about 30 - 100 mm and a shorter diameter of about 20 - 30 mm or other shape in accordance with the purpose of use, to obtain a coke having high quality and high strength from a weakly caking or noncaking coal. Ordinarily, good results can be obtained by blending 3 to 4 kinds or at most 7 to 8 kinds of starting coals, in the same manner as in the usual caking processes of caking or strongly caking coals. The coal briquette thus obtained is used for the production of cokes, either alone or after having incorporated therein one third, based on the total weight, of a caking or strongly caking coal. Accordingly, the composition ratio of the starting coal mixture is so selected, according to the empirical rule and the results of preliminary experiments, that the volatile matter content, dilatation degree, and the fluidity, etc. fall into the desirable ranges. The balance between various properties such as ash content, sulfur content and the like, should also be taken into consideration.
Said caking agent is employed for the purpose of giving the weakly caking coal not only an adhesive character but also the caking power necessary for forming a coal briquette. There have hitherto been disclosed many types of caking agents, most of which have been obtained through a treatment of tars or pitches. They are roughly classified into petroleum type and coal type. A few examples of caking agents commercially available are shown below:
Petroleum type: P. D. A. (propane-deasphalted asphalt), S. D. A. (solvent-deasphalted asphalt), S.A. (straight asphalt), K. R. P. (Kreba Reginus pitch), A. S. P. (asphalt pitch) and natural asphalt;
Coal type: pitch (high, medium, low), tar, creosote oil, anthracene oil.
In addition to the above, it is also effective to use a petroleum type binder in combination with a coal type one in a certain definite proportion.
A variety of caking agents are used depending on the nature and combination of the starting coals used. The quantity of caking agent is usually in the range of 6 to 10% by weight based on the total coal.
Some of the caking agents are liquids, but the others are solids which melt at the kneading temperature. A caking agent is added to a finely pulverized compounded coal and kneaded with them so that the caking agent spreads over the pulverized coal well, coats the surface of said coal and adheres to the coal. This can be realized only in the presence of a certain quantity of water in the kneaded mixture. In other words, a petroleum type or coal type of caking agent can spread satisfactorily on the surface of finely devided coal only in the presence of water. A satisfactory results can be obtained by effecting the kneading in the presence of about 8 to 14% by weight of water, including the water originally contained in the starting coal. An excessive quantity of water is undesirable because it condenses and remains in the coal briquette. In the conventional processes, the water is fed by direct blowing of steam into the kneader which steam heats the kneader at the same time. The kneader rotates as low as 17 to 20 r.p.m. and this means the use of an excessive steam.
In general, starting coals contain about 8 to 14% water, because they have been stored outdoors. However, the product, raw coal for coal briquette preferably contains about 5 to 8% of water. Therefore, it is necessary to release the excessive water in the course of kneading and leave a desirable water content behind in the coal. Nevertheless, the excessive water tends to form condensed water in the conventional processes, particularly in the wet processes. Thus, the aforesaid desirable water content is difficult to realize so far as the conventional processes are concerned.